Cross-flow filtering process for separating fluid from a free-flowing medium and installation for implementing it

ABSTRACT

For thickening a retentate by means of membrane filtration (ultra-/micro-filtration) to obtain the highest possible wet sludge proportion, a gaseous medium is admixed with the retentate. To this end an open feed line (13) above the retentate level (14), a porous ring (17) in the retentate, or a simple line connection (19, 20, 28, 29) in a tank (6) are proposed. A number of advantages accrue as a result of the lowering of the viscosity of the retentate achieved in this way, such as savings of energy for retentate recirculation, efficient installations because of longer series or passes of filtration modules, and savings in squeezing installations because of the direct filtration with high solid components.

FIELD OF THE INVENTION

The invention relates to a cross-flow filtration method for separatingliquid from a flowable medium, wherein a material stream to be filteredcontains parts to be separated and is moved in as a retentate through atleast one module containing porous membranes and a filtrate is separatedtherefrom, as well as to an installation for performing the method.

BACKGROUND OF THE INVENTION

A method of this type is already known from EP-B1-0 427 099 (W. Muller).It achieves the purpose of obtaining a predetermined degree ofthickening of the retentate in an operationally assured and simplemanner. This micro-filtration method, in which the material stream to befiltered passes under pressure in a circuit through a micro-filtrationmodule with membranes, over which the stream passes tangentially withcontinuous addition and removal of concentrate, operates by measuringthe velocity and pressure drop in the retentate stream. Theconcentration of a turbid material is calculated from these measuredvalues and the concentrate output is adjusted in case of deviations froma set value.

The aim in connection with such membrane filtrations in the form ofultra-filtration or micro-filtration is to thicken the retentate in thecourse of the thickening process to obtain the highest possibleproportion of wet sludge. By means of this it is possible to reduce thecosts for subsequent drying and disposal of the retentate. The degree ofretentate thickening which can be achieved depends essentially on thehighest viscosity at which the retentate can still be passed through themembrane modules of the installation used. To achieve high viscosity, ithas been known up to now to keep at low levels the number of modules ineach series (during each pass) and the velocity of the retentate overthe membranes. However, a reduction of the number of modules per passreduces the output of the installation and thus decreases it efficiency.

SUMMARY OF THE INVENTION

It is therefore the object of the invention to recite a method and aninstallation for the separation of liquids from a flowable medium, whichmake it possible to separate more liquids in comparison with knownmethods and in this way to obtain higher proportions of wet sludge inthe retentate.

In accordance with the invention the object of the invention is attainedby means of a method of the type mentioned at the outset by admixing agaseous medium to the retentate. Various practical embodiments of thismethod ensue from the claims. Installations for the advantageousexecution of the method in a filtration installation are also recited inthe claims.

Tests with an installation in accordance with the invention have shownthat it is possible to provide thickening of up to 90 to 95% parts ofwet sludge by doubling the customary number of filtration modules perpass. With increased proportions of wet sludge, this is assumed to bethe result of a reduction in the viscosity of the retentate because ofthe admixture of air. The novel method permits the employment of alarger number of filtration modules in series per pass and thus moreefficient installations. Because of the larger proportions of wet sludgein the retentate there is an energy savings during final drying. Inaddition, the lowering of the viscosity of the retentate brings areduction in the membrane wear. The mentioned advantages also occurduring juice separation from fruit mash in membrane installations knownper se, if the mash with all solid parts is directly filtered by meansof the cross-flow method, while omitting the squeezing process.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are described in detail in thefollowing description and the drawing figures. Shown are in:

FIG. 1, a diagram of an installation in accordance with the inventionfor separating liquid from a medium, with different devices for admixingair with the medium;

FIG. 2, the admixture of air with the surface of a retentate in a batchtank,

FIG. 3, the admixture of air with the retentate in a line by means of aperforated pipe,

FIG. 4, the admixture of air with the retentate from the permeate sideof a filtration module in a series of modules placed downstream fromeach other, and

FIGS. 5 to 13, devices for admixing gas to a medium in an installationin accordance with FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 schematically represents a module 1 of a type known per se,having a porous membrane for separating a permeate from a retentate. Theretentate comprises liquid and non-liquid portions, which are providedto the module 1 via a line 3. In accordance with the cross-flow method,the retentate flows across the surface of the membrane 2 in the module1, wherein liquid portions move through its pores and are removed aspermeate, as indicated by the arrow 4. Fruit mash from which the juiceis to be separated, as well as waste water sludge which is to bedewatered, are known as important examples of use, but also fruitjuices, which are intended to be clarified. Such known filtrationinstallations comprise a multitude of such modules 1, which aresimultaneously arranged parallel as well as in series in so-calledpasses in order to increase the filtration output and to reduce theproportion of remaining liquid in the retentate.

As shown by FIG. 1, the retentate has been guided in a retentate circuitthrough the line 3 and the module 1. The retentate enters this retentatecircuit as a flowable medium via a feed line 5 and a batch tank 6. Inso-called batch operation, the batch tank 6 is filled with a charge or"batch" of the medium to be separated and the batch is retained in theretentate circuit during the subsequent filtering operation until adesired degree of dewatering or wet sludge proportion has been achieved.Only then is the retentate removed from the circuit via a line 7 and athrottle 22, in the course of which a rinsing liquid for cleaning theretentate circuit is introduced through the feed line 5.

From the batch tank 6 the retentate reaches in a manner known per se theline 3 and the module 1 by way of a controlled conveying pump 8, a flowmeter 9 and a static mixer 10. All volumetrically conveying pumps, whichare known to operate by means of cells, wobble shafts, gear teeth orpistons, are suitable as conveying pumps 8. From the direction of themodule 1 the retentate again reaches the batch tank 6 through a furtherline 11 and a further mixer 12, and the retentate circuit is closed withthis. A control stage 24 is provided in a manner known per se forregulating the flow in the retentate circuit. As a function of themeasuring signals of the flow meter 9 it controls the pressure at theinlet of the module 1 by means of a servo valve 23, and the flow speedby means of the conveying pump 8. The control stage 24 further controlsor regulates the process circuit as a function of the signals of aviscosity sensor 25 at the inlet of the module 1 and two pressuresensors 26, 27 at the inlet and at the outlet of the module 1. Thementioned values of flow speed, viscosity and pressure of the retentatechange as a result of thickening and therefore require regulation.

As shown by FIG. 1, various devices for admixing a gas, preferably air,to the retentate are provided for improving the conveyability of theretentate in the circuit. The return line 11 terminates at the batchtank 6 with an opening 13 at a defined height above the level 14 of theretentate. The retentate falls out of the opening 13 down to the level14 and on striking the flow edge at a high speed entrains air ornitrogen from the tank 6 in the medium, as shown in detail in FIG. 2. Alevel sensor 15 for keeping the level 14 constant has been mounted onthe tank 6 to assure a stable operation.

It has been shown that the admixture of gas during the free fall of theretentate in the batch tank 6 takes place particularly effectively ifthe diameter of the tank 6 is narrow and the height of the level 14 inthe tank 6 is low. The rising speed of the gas bubbles introduced intothe retentate is determined by their size and the viscosity of theretentate.

So that these gas bubbles reach the connecting line 18 between the batchtank 6 and the conveying pump 8, the flow speed of the retentate on theway down must be greater than the rising speed of the gas bubbles. Inconnection with the mentioned requirements for the tank diameter and thelevel height, a downward flow at increased flow speed forms in theretentate, which therefore effectively pulls the gas bubbles into theline 18. Accordingly, an open line in the lower zone of the tank 6 inthe area of swift downward flow is sufficient for the introduction ofair, perforated pipes etc. are not necessary.

As FIG. 1 shows, the batch tank 6 is embodied in a closed form. Thecomposition of the gas atmosphere above the level 14 can be selected. Itis determined by means of a feed 16 for fresh gas to replace the admixedgas. A porous ring 17 with an external gas supply is disposed in theretentate in the batch tank 6 below the surface level 14 as a furtherdevice for admixing gas. The gas can also be easily admixed in theconnecting line 18 between the batch tank 6 and the conveying pump 8 viaa line 19, or in the return line 11 via a line 28 or 29. A similar gassupply is provided between the flow meter 9 and the static mixer 10 viaa line 20 and a valve 21 controlled by the control stage 24.

The simple admixture of gas via the lines 19, 20, 28, 29 makes the useof mixers 10 and 12 for homogenizing the retentate practical, in orderto achieve a homogeneously reduced viscosity. Such mixers areunnecessary if a homogeneous distribution is already provided whenadmixing the gas to the retentate circuit, for example by the use ofperforated lines, sintered lines, membranes or Venturi nozzles in theretentate line.

FIG. 3 shows by way of example the admixture of air 30 in the line 11 inaccordance with FIG. 1 through the line 28, which is embodied as aperforated pipe 28' inside the line 11. The holes 31 are disposed in thepipe 28' on the side facing away from the retentate stream, as indicatedby the arrows 32.

As already mentioned, industrial filtration installations comprise aplurality of filtration modules in series. Such a series of only fourmodules 41, 41' is shown in FIG. 4 by way of example. The series 41, 41'has been used in place of the module 1 in FIG. 1 between the feed line 3and the return line 11 for the retentate. Each module 41, 41' has itsown return line 44, 44' for the permeate, which are brought together ina permeate collection line 44". One of the modules, suitably the firstone 41' of the series, can advantageously be used for admixing gas,which is introduced under pressure from the permeate side of thefiltration membrane (2 in FIG. 1) through the line 44'. In this case theline 44' is separated from the permeate collection line 44" by means oftwo blocking valves 45, 46 and is connected with a gas supply 47, asshown in FIG. 4.

The gas supply can be shut off by means of the blocking valves 45, 46and the module 41' can be operated in a normal filtering operation.Because of the fine and evenly distributed membrane openings, theadmixture of gas to the retentate through the filtration membrane of amodule 41' results in a particularly homogeneous gas distribution in theretentate.

FIGS. 5 to 13 show variants of the devices for gas admixing in the batchtank 6 in accordance with FIG. 1. An open batch tank 6 is employed inthe examples in accordance with FIGS. 5 to 9, so that ambient air isadmixed as the gas. As shown by FIG. 5, the retentate return line 11terminates in a spray head 50 above the retentate level 14 in the tank6'. High speed is imparted to the partial retentate flows when they exitthe spray head 50, in order to entrain a desired amount of air whenstriking the surface 14.

In accordance with the variant of FIG. 6, the spray head has beenreplaced by a deflector plate 60 at the outlet of the retentate line 11,and in accordance with FIG. 7 by an atomizer 70 with mechanically movedelements. In these examples the admixture of air to the retentatealready takes place above the retentate level 14 and no high impactspeed is attained here.

FIG. 8 shows a batch tank 6', in which a sintered plate 80 has beendisposed below the retentate level 14 as the membrane for admixing air,similar to the porous ring 17 in FIG. 1. In the arrangement inaccordance with FIG. 9, the retentate line 11 has a membrane 90 ahead ofits end above the retentate level 14 in the tank 6', through which airis admixed under pressure. The membrane 90 can be arranged in amicro-filtration module or as a sintered pipe.

FIG. 10 shows a closed batch tank 6, which has its own retentate circuit101 with a recirculating pump 102 for the introduction of gas above theretentate level 14. In this case the retentate return line 11 isconnected with the retentate in the tank 6 below the level 14. FIG. 11shows how the retentate is distributed from the return line 11 to anumber of free line ends 110 even in a closed tank 6. The retentateexiting here absorbs the gas which is present above the surface 14 ofthe retentate. FIG. 12 shows an arrangement with an effect similar toFIG. 11, only here the retentate from the line 11 is introduced into theclosed batch tank 6 above a perforated plate 120, which distributes itover the gas chamber located underneath it.

FIG. 13 again shows a closed batch tank 6 with a stirrer 130 disposedtherein for high rpm operation. The stirring members of the stirrer 130simultaneously pull the gas above the retentate level 14 down into theretentate and there distribute it evenly for further transport in thecircuit via the line 18.

The gas admixture in accordance with the invention has furtheradvantages besides the lowering of the viscosity of the retentate inaccordance with the invention. In connection with liquefied fruit mashor squeezed fruit juice, it is known to oxidize phenols by means of airadmixed in the tanks or by admixed oxygen 6 for improving the juicestability against later cloudiness. In the course of employing themethod of the invention for these purposes, this effect appears in to agreater degree because of the admixture and distribution of suitablegases.

The filtration output in the modules 1 is furthermore improved becausethe reduction in viscosity advantageously affects the shear forces ofthe permeate stream at the membrane surfaces. Besides the increaseddegree of thickening of the retentate, the results also are energysavings because of reduced recirculating work for the less viscousretentate, a reduction in membrane wear and the option of employing moreefficient filtration installations, because with less viscous retentatesit is possible to operate a larger number of filtration modules inseries.

Known squeezing methods in connection with fruit mash can be omitted,because this mash can now be directly filtered by means of thecross-flow method in spite of all the solids contained in it. If theadmixture of air is sufficiently finely distributed, the retentateassumes an almost foam-like state. But an improvement is alreadyachieved if the distribution is omitted and the gaseous medium is addedto the retentate stream downstream of the recirculating pump simply in aseries of timed interruptions.

I claim:
 1. A cross-flow filtration method for thickening a solid-liquidmixture, comprising moving the mixture to be filtered in the form ofretentate through at least one membrane module (1, 41) with porousmembranes (2) to separate liquid filtrate therefrom through saidmembrane, and reducing the viscosity of said retentate to facilitate themovement thereof through said at least one membrane module by admixing agaseous medium with the retentate, said gaseous medium being onlyadmixed with the retentate if the viscosity of the retentate lies abovea preset value.
 2. The method in accordance with claim 1 carried out ina batch operation wherein the retentate is recycled in a circuit to abatch tank (6), and the gaseous medium is admixed to the retentate inthe supply flow (3) to the membrane modules (1, 41).
 3. The method inaccordance with claim 2, wherein the gaseous medium is admixed with theretentate in the batch tank (6) in the course of its return into thebatch tank (6) by impinging the retentate with the greatest possiblespeed on the surface (14) of the retentate already present in the tank(6).
 4. The method in accordance with claim 2, wherein the flow speed inthe retentate circuit is changed as a function of the average amount ofthe gaseous medium admixed with the retentate per unit of time.
 5. Themethod in accordance with claim 4, wherein the flow speed is changed bythe conveyed amount of retentate in such a way that the pressure drop inthe retentate circuit over the membrane modules (1, 41) or the inletpressure at the first module remain constant.
 6. The method inaccordance with claim 1, wherein the amount of the admixed gaseousmedium is regulated as a function of the density or viscosity of theretentate or its pressure drop at said at least one membrane module. 7.The method in accordance with claim 1, wherein the retentate is suppliedto the membrane modules (1, 41) by means of a pump (8) through a line(3), and that the gaseous medium is admixed with the retentate streamdownstream of the pump (8) in such a way that gas bubbles of a size upto the diameter of the feed line (3) are created.
 8. The method inaccordance with claim 1, wherein the retentate is supplied to themembrane modules (1, 41) by means of a pump (8) through a line (3), andthe gaseous medium is admixed with the retentate stream downstream ofthe pump (8) in a series of timed interruptions.
 9. The method inaccordance with claim 1, wherein the solid-liquid mixture to be filteredis passed through a series of modules (41, 41') arranged in series onebehind another and having porous membranes, and gas is admixed with theretentate under overpressure from the filtrate side through the membranein one of the first or the last module (41') of the series.
 10. Aninstallation for thickening a solid-liquid mixture to a high solidscontent, comprising at least one filtration module (1, 41) with porousmembranes (2), over which a stream of the solid-liquid mixture to befiltered can be passed tangentially as the retentate by a conveying pump(8) and a feed line (3) to the filtration modules (1, 41), and a device(13, 17, 19, 20) for reducing the viscosity of said retentate tofacilitate the movement thereof through said at least one filtrationmodule by admixing a gaseous medium with the solid-liquid mixture to befiltered, wherein said device for admixing (20) comprises a gas supplyline, in which a regulating element (21) for the amount of gas suppliedis arranged, and wherein said regulating element (21) for the amount ofthe supplied gas is operatively connected with a device for measuringviscosity (25), pressure (26, 27) or flow-through (9) of the retentate.11. The installation in accordance with claim 10, additionallycomprising a homogenizer or a static or articulated mixer (10, 12)disposed downstream of the device for admixing (20, 29) in the line (3,11) for the retentate.
 12. The installation in accordance with claim 10,including a retentate circuit with a batch tank (6, 6') located in itfor the mixture to be filtered, said retentate circuit having a returnline (11) the end (13) of which is arranged in the batch tank (6, 6')above the retentate level (14) in such a way that the retentate from thereturn line (13) strikes the retentate level (14) directly.
 13. Theinstallation in accordance with claim 10, including a retentate circuitwith a batch tank (6, 6') located in it for the mixture to be filtered,said retentate circuit having a return line (11) the end of which isarranged in the batch tank (6, 6') above the retentate level (14), andnozzles, spray heads (50), deflector plates (60), mechanically drivenatomizers (70), multi-armed distributors (110) or perforated bottoms(120) inserted into the tank (6), are disposed at the outlet of theretentate line (11) for admixing the gas located above the retentatelevel (14) with the medium to be filtered.